Non-volatile memory systems, such as flash memory, have been widely adopted for use in consumer products. Flash memory is used in numerous applications including mobile phones, digital cameras, MP3 players and many other applications. Flash memory may be found in different forms, for example in the form of a portable memory card that can be carried between host devices or as a solid state disk (SSD). In order to be cost efficient, it is desirable to implement SSD memories using high density Multi-Level Cell (MLC) flash memories rather than Single-Level Cell (SLC) flash memory which has 1-bit per cell storage capacity. Using MLC technology, the memory capacity can be increased by a factor of 2, 3 or more (for memories storing 2, 3 or more bits in each cell, respectively) as compared to SLC flash without increasing the physical dimensions of the memory. What increases in MLC flash is the amount of data stored in each memory cell and in each physical page (Word Line (WL)) of the memory.
Data stored in flash memory is susceptible to corruption due to various factors. Data in flash memory may be corrupted simply by ordinary read, program and erase sequences, where memory cells in physical proximity to those that are subject to a read, program or erase action may be unintentionally stressed to a point where stored charge levels are altered enough such that bit errors are induced in those memory cells. With respect to read operations, read stress is induced not only on the word line being read but also when the read voltage is applied to other word lines in a physical memory block. The stress on word lines not being read is sometimes referred to as a read disturb effect. If a block is being read unevenly, some pages may be corrupted without being read. Error correction codes (ECC) are typically implemented when storing and retrieving data in order to improve the chance that the data can be read from flash memory without errors. When the corruption is beyond the ability of the ECC algorithm to repair, however, the system may completely lose the data in those pages.
Prior attempts to address read disturb effects typically involve a read scrub procedures where data is copied from one block to another block so that the ECC algorithm used by the flash memory device can attempt to correct accumulated errors in the block during the copy process. Some existing read scrub processes are automatically performed at regular intervals, where a block will be copied after a fixed number of read operations and the flash memory device tracks and stores multiple counters for each of the blocks. This approach may require that the flash memory device use processing and storage resources to maintain and update the various counters. This approach may also potentially lead to performing frequent and unnecessary copying operations that can affect performance and life span of the flash memory device.